Christine Ouyang

A new inorganic EUV resist with high-etch resistance

Performance requirements for EUV resists will necessitate the development of entirely new resist platforms. As outlined in the ITRS, the new resists for EUVL must show high etch resistance (to enable pattern transfer using thinner films), improved LER and high sensitivity. A challenge in designing these new resists is the selection of molecular structures that will demonstrate superior characteristics in imaging and etch performance while maintaining minimal absorbance at EUV wavelengths. We have previously described the use of inorganic photoresists in 193 nm and e-beam lithography. These inorganic photoresists are made of HfO2 nanoparticles and have shown etch resistance that is 25 times higher than polymer resists. The high etch resistance of these materials allow the processing of very thin films (< 40 nm) and will push the resolution limits below 20 nm without pattern collapse. Additionally, the small size of the nanoparticles (< 5 nm) leads to low LER while the absorbance at EUV wavelengths is low. In this presentation we show that these inorganic resists can be applied to EUV lithography. We have successfully achieved high resolution patterning (<30 nm) with very high sensitivity and low LER.

Oxide nanoparticle EUV resists: toward understanding the mechanism of positive and negative tone patterning

DUV, EUV and e-beam patterning of hybrid nanoparticle photoresists have been reported previously by Ober and coworkers. The present work explores the underlying mechanism that is responsible for the dual tone patterning capability of these photoresist materials. Spectroscopic results correlated with mass loss and dissolution studies suggest a ligand exchange mechanism responsible for altering the solubility between the exposed and unexposed regions.

Non-aqueous negative-tone development of inorganic metal oxide nanoparticle photoresists for next generation lithography

As feature sizes continue to shrink, the need for new materials and processes becomes more urgent. In order to achieve high-resolution patterns and low line edge roughness (LER), there have been many studies on small molecular resists. In terms of processes, there have been growing interests in negative-tone development because of its better performance in printing narrow trenches and contact holes. As new patterning materials, we have synthesized inorganic nanoparticle resists that consist of a metal oxide (HfO2 or ZrO2) core surrounded by organic ligands. The inorganic core provides high etch-resistance while the organic ligands give the resists photochemical functionality. Because of their high etch-resistance, thin films of these nanoparticle photoresists are sufficient to provide good pattern transfer to the substrate and eliminate problems such as pattern collapse. Negative-tone patterning of these nanoparticle photoresists can be achieved by using an organic solvent. The small sizes (1-3nm) of these nanoparticle resists can also enable high-resolution patterning and have the potential to reduce LER. We have successfully shown negative-tone patterning of these nanoparticle resists with features as small as 30 nm using both e-beam and EUV lithography and this paper seeks to study the NTD results with different negative-tone developers.

Negative-tone development of photoresists in environmentally friendly silicone fluids

The large amount of organic solvents and chemicals that are used in todays microelectronic fabrication process can lead to environmental, health and safety hazards. It is therefore necessary to design new materials and new processes to reduce the environmental impact of the lithographic process. In addition, as the feature sizes decrease, other issues such as pattern collapse, which is related to the undesirable high surface tension of the developers and rinse liquids, can occur and limit the resolution. In order to solve these issues, silicone fluids are chosen as alternative developing solvents in this paper. Silicone fluids, also known as linear methyl siloxanes, are a class of mild, non-polar solvents that are non-toxic, not ozone-depleting, and contribute little to global warming. They are considered as promising developers because of their environmental-friendliness and their unique physical properties such as low viscosity and low surface tension. Recently, there have been emerging interests in negative-tone development (NTD) due to its better ability in printing contact holes and trenches. It is also found that the performance of negative-tone development is closely related to the developing solvents. Silicone fluids are thus promising developers for NTD because of their non-polar nature and high contrast negative-tone images are expected with chemical amplification photoresists due to the high chemical contrast of chemical amplification. We have previously shown some successful NTD with conventional photoresists such as ESCAP in silicone fluids. In this paper, another commercially available TOK resist was utilized to study the NTD process in silicone fluids. Because small and non-polar molecules are intrinsically soluble in silicone fluids, we have designed a molecular glass resist for silicone fluids. Due to the low surface tension of silicone fluids, we are able achieve high aspect-ratio, high-resolution patterns without pattern collapse.

Environmentally friendly patterning of thin films in linear methyl siloxanes

A number of green solvents have been explored to reduce the environmental impact of many chemical processes. Among them, linear methyl siloxanes make up a class of solvents that are low in toxicity, VOC exempt and not ozone-depleting. In addition, their unique physical properties such as low surface tension and low viscosity can mitigate several issues encountered with conventional processing solvents. In order to understand the behavior of linear methyl siloxanes as processing solvents, the solubilities of polymers and small molecular glasses are studied in this work. Using lithography as a test of solubility differences, we have successfully shown patterning of thin films in linear methyl siloxanes thereby demonstrating their utility in processing organic systems.

Environmentally friendly natural materials-based photoacid generators for next-generation photolithography

We describe the development of new triphenylsulfonium photoacid generators (TPS PAGs) with semifluorinated sulfonate anions containing glucose or other natural product groups, and their successful application to patterning sub-100 nm features using 254 nm and e-beam lithography. The TPS PAGs with functionalized octafluoro-3-oxapentanesulfonate were synthesized efficiently in high purity and high yield by utilizing simple and unique chemistries on 5-iodooctafluoro-3-oxapentanesulfonyl fluoride. The PAGs has been fully evaluated in terms of chemical properties, lithographic performance, environmental friendliness or toxicological impact. The PAGs are non-toxic and it is susceptible to chemical degradation and to microbial attack under aerobic/anaerobic conditions. These new PAGs are very attractive materials for high resolution photoresist applications and they are particularly useful in addressing the environmental concerns caused by PFOS and other perfluoroalkyl surfactants.

Solvent development processing of chemically amplified resists: chemistry, physics, and polymer science considerations

Solvent development of chemically amplified (CA), negative tone photoresists depends on several factors including molecular weight of the photoresist, the strength of polymer-solvent interactions, and the strength of polymer-polymer interactions in the undeveloped regions. Absent are the ionic interactions present in the aqueous base development of CA resists that greatly aids dissolution and image contrast. In its place, strong hydrogen bonding of the exposed photoresist leads to effective resistance to dissolution in non-polar developers. These effects are discussed in the context of Flory- Huggins theory. As part of a study of low environmental impact developers several, non-polar solvents have been investigated with negative tone, chemically amplified photoresists. These include supercritical CO2, hydrofluoroethers and silicone fluids. Each of these solvents has low surface energy, unique dissolution characteristics and is capable of developing sub-50 nm patterns. Performance aspects of these developers will be described.

Patterning conventional photoresists in environmentally friendly silicone fluids

The chemical waste generated in todays microelectronic fabrication process can be released into the environment and cause environmental and health concerns. It is therefore necessary to develop an environmentally friendly process that can eliminate the use of toxic chemical solvents. Silicone fluids are linear methyl siloxanes that only contain carbon, hydrogen, oxygen and silicon. They are low in toxicity, not ozone-depleting and contribute little to global warming. They degrade into naturally occurring compounds instead of accumulating in the atmosphere and can be recycled. Their unique physical and chemical properties have also made them promising developers for lithography. For example, their low surface tension can eliminate pattern collapse problems associated with high aspect-ratio features. Silicone fluids are non-polar solvents and their solvent strength is weaker than that of saturated hydrocarbons but stronger than that of the commercially available saturated hydrofluorocarbons and may be enhanced by adding other solvents. Two conventional photoresists used in this study, PBOCST and ESCAP are both insoluble in silicone fluids before or after exposure. However, the solubility of PBOCST and ESCAP in silicone fluids can be increased by using a silicon-containing additive. In this paper, we demonstrate this novel and environmentally friendly development of conventional photoresists in silicone fluids.

Environmentally friendly processing of photoresists in scCO2 and decamethyltetrasiloxane

The chemical waste generated in todays microelectronic fabrication processes has driven the need to develop a more environmentally benign process. Supercritical CO2 (scCO2) has been evaluated as an environmentally friendly solvent for photoresist development. It is nontoxic, nonflammable, and inert under most conditions. It also possesses advantages such as liquid-like densities, gas-like diffusivity, and zero surface tension. Although scCO2 is a poor solvent for most polymers, certain fluorine-and silicon-containing polymers have shown solubility in scCO2. Previously, negative-tone patterns of 100nm have also been developed in scCO2 using conventional photoresists such as ESCAP and PBOCST with the aid of fluorinated quaternary ammonium salts (QAS). However, the incorporation of fluorine degrades plasma etch resistance, and because of their persistence in nature, fluorinated compounds are coming under increased scrutiny. In order to make the process more environmentally benign, the elimination of fluorine is desirable. Some molecular glass photoresists without the incorporation of fluorine and silicon have thus been designed and synthesized to be processed in scCO2. In addition to scCO2, another environmentally friendly, low VOC solvent, decamethyltetrasiloxane has also been investigated to develop conventional photoresists. In this paper, we demonstrate the patterning of photoresists in both scCO2 and decamethyltetrasiloxane.